PTC resistances or PTC thermistors have a positive temperature coefficient (PTC) of electrical resistivity and are electrically conductive materials which have better electrical conductivity at low temperatures than at higher temperatures. Within a relatively narrow temperature range, the electrical resistivity rises markedly with increasing temperature. Materials of this type can be used for heating elements, current-limiting switches or sensors. Known PTC polymer compositions have low resistance at room temperature, i.e. at about 24° C., thus allowing electrical current to flow. When temperature is increased greatly, up to the vicinity of the melting point, resistance increases to a value that is from 104 to 105 times the value at room temperature (24° C).
Polymeric PTC compositions consist of a mixture of organic polymers, in particular crystalline and semi-crystalline polymers, with electrically conductive additives. The PTC effect in the prior art is mostly based on structural alteration of crystalline polymer domains during temperature increase to give amorphous or less crystalline domains. Specific polymer mixtures comprise not only the thermoplastic polymers but also thermoelastic polymers, resins and other elastomers. An example of this is described in WO2006115569.
Polymer compositions of the above type have the disadvantage that the PTC effect is restricted to a switching behavior based on structural alteration of the polymers used as main component. PTC intensity, i.e. the change of resistance, is moreover very highly dependent on the polymer or polymer blend used.
The prior art also discloses liquid polymer dispersions with PTC effect which are provided for coatings or lacquer systems. The PTC effect in these liquid polymer dispersions is based on an additive, for example paraffin or polyethylene glycol (PEG), see for example WO 2006/006771.
JP 2012-181956 A discloses an aqueous paint dispersion which comprises an acrylate copolymer, a crystalline, heat-curing resin, paraffin, carbon black and graphite as electrically conductive material, and also a crosslinking agent. The heat-curing resin is preferably a polyethylene glycol and the crosslinking agent is preferably a polyisocyanate. The paint is applied to a surface and heated for from 30 to 60 min to a temperature of from 130 to 200° C. A coating is thus produced which has PTC effect and which can serve as planar heating element.
Impregnation compositions and coating compositions of the above type are problematic because uncontrolled loss of solvent by evaporation often occurs during application, with formation of craters and blisters that are visible to a greater or lesser extent in the coating. If pretreatment of the substrate to be coated is inadequate, adhesion of the coating is often defective because of excessively low or excessively high surface energy, or else unsuitable surface structure. This results in break-away and flaking of the functional layer and, associated therewith, considerable impairment of electrical conductivity and of the PTC effect. Defective application of the impregnation composition or coating composition, inadequate drying and/or crosslinking, excessively high drying temperatures or hardening temperatures, excessively long drying times or hardening times, or an excessive dose of crosslinking radiation, have a direct adverse effect on the durability and functionality of the coating. This is true in particular, but not only, in the coating of textiles. Another phenomenon often encountered, either to a relatively small degree or over a large area, is “bleeding” of paraffin from said impregnation systems and coatings, causing failure of same after a short service time.
The article by M. Bischoff et al. “Herstellung eines Black-Compounds aus PE/LeitruB zur Anwendung für aufheizbare Fasern” [Production of a black compound material form PE/conductive carbon black for use for heatable fibers] in Technische Textilien 2/2016, pp. 50-52 relates to the electrical conductivity of, and the generation of heat by, a compound material made of 90% of polyethylene and 10% of conductive carbon black.
U.S. Pat. No 6,607,679 B2 describes an organic PTC thermistor which comprises a low-molecular-weight organic compound, electrically conductive metal particles, and a matrix made of at least two polymers, where the surface of each conductive particle has from 10 to 500 conical projections. About 10 to 1000 of said particles can have been bonded in the form of a network to give a secondary particle. The individual particles preferably consist of nickel. Their average diameter is about 3 to 7 μm. At least one of the two polymers in the matrix must be a thermoplastic elastomer. The thermoplastic elastomer ensures reproducibility of the electrical properties of the PTC composite material, in particular low electrical resistance at room temperature and large resistance change at elevated temperatures, even when the low-molecular-weight organic compound melts. The low-molecular-weight organic compound is preferably a paraffin wax with melting point from 40 to 200° C. The matrix can comprise other electrically conductive particles, for example made of carbon black, graphite, carbon fibers, tungsten carbide, titanium nitride, titanium carbide or titanium boride, zirconium nitride or molybdenum silicide. The PTC thermistor can be produced via pressing at elevated temperature (for example at 150° C.) or via application of a mixture which additionally comprises a solvent such as toluene to a carrier, for example a nickel foil, and then heating and crosslinking of the resultant coating.
WO 2006/006771 A1 describes an aqueous electrically conductive polymer composition which has a positive temperature coefficient (PTC). It comprises a water-soluble polymer, a paraffin, and also electrically conductive carbon black. The water-soluble polymer is preferably polyethylene glycol. The aqueous composition can be used to produce coating which can be used as flat heating element.
The materials disclosed in the prior art for the production of electrically conductive polymer moldings with positive temperature coefficient (PTC) are based on aqueous dispersions and are unsuitable for processes involving melting, for example extrusion, melt spinning and injection molding. Compositions for electrically conductive polymer moldings with PTC for the purposes of this invention comprise, as substantial constituents, a matrix polymer, a conductivity additive and a phase-change material. The processing temperature in processes involving melting is usually in the range from 100° C. to above 400° C., in particular in the range from 105° C. to 450° C. At these temperatures, the phase-change material is liquid and has low viscosity. In contrast, the viscosity of the plastified matrix polymer is substantially higher, sometimes higher by several orders of magnitude. Even when there is good miscibility of matrix polymer and phase-change material, for example polyethylene and paraffin, the phase-change material takes the form of a phase intercalated in the matrix polymer. When the high mechanical load or high shear stress, or pressure, at extruder dies or injection-molding nozzles is combined with a temperature well above the melting range of the phase-change material, the result is that the intercalated low-viscosity phase-change material is forced out of the matrix polymer and to some extent lost into the environment. This effect can moreover be amplified in particular temperature-shear stress/pressure ranges by deformation-induced phase segregation or demixing. Loss of phase-change material is particularly large when the dimension of the extruded molding, for example a fiber or foil, is small in at least one spatial direction: less than 1000 μm. For the purposes of the present invention, the term “bleed-out” is also used for the loss of phase-change material.
During the intended use of the PCT molding, the phase-change material is subsequently heated and liquefied, sometimes with exposure to considerable mechanical load. “Bleeding” of the phase-change material therefore also occurs during the use of the PTC molding.